```js
// Button.stories.js

import Button from './Button.svelte';
import MarginDecorator from './MarginDecorator.svelte';

export default {
  title: 'Button',
  component: Button,
  decorators:  [() => MarginDecorator],
};

// Your templates and stories here. 
// Don't forget to use the component you're testing and not the MarginDecorator component

```

```html
<!-- MarginDecorator.svelte -->

<div>
  <slot/>
</div>

<style>
  div { 
    margin: 3em;
  }
</style>
```